Solid, thermoplastic polymers of lactone esters have been known in the art for some time. U.S. Pat. No. 3,169,945 describes a method for producing such polymers whereby a molar excess of a lactone monomer is reacted with an active-hydrogen containing initiator that is capable, with or without the aid of a catalyst, of opening the lactone ring and adding it as an open chain without forming water of condensation. The solid polymers which can be produced by this method are highly crystalline and display sharp melting points.
Typical uses of the thermoplastic polylactone polymers include their use as a material of construction for orthopedic casts, splints, and similar devices, as described in U.S. Pat. No. 3,692,023; and their use as plasticizers for vinyl resin compositions, as described in U.S. Pat. No. 3,592,877.
The rheological characteristics of the solid lactone polymers have proven to be an obstacle to their more widespread use in certain applications. The melt index of the polymers rises sharply as the temperature is increased above the melting point. Except at quite high molecular weights (i.e., reduced viscosity greater than about 0.8), the polymer is difficult to extrude in sheet form, since the molten extrudate has poor dimensional stability. Costly chill roll equipment must generally be employed to reduce the temperature of the extruded sheet as quickly as possible upon emergence from the sheet die. In blown film applications, the extrudate usually will not form a self-supporting film bubble at reduced viscosities below about 1.5. It is very difficult to produce such high viscosity material in conventional commercial polymerization equipment.
The rheological properties of the lactone polymers have hindered their more widespread acceptance in the field of orthopedic casts, despite the fact that the casts produced from them have excellent properties, i.e. they are lightweight, tough, rigid, water and soil resistant, and easily removed. The polymer must be heated above its melting point in order to apply it as an orthopedic cast material. At the application temperature, the polymer tends to be more fluid than is desired and, therfore, difficult to handle and apply.